In the field of relief printing such as letter press, flexo and the like, a computer plate making technique (computer to plate (CTP) technique), also known as a digital image formation technique, is becoming extremely general nowadays. In the CTP technique, a photographic mask (also called a photomask or negative film) conventionally used for covering an area unwanted to be polymerized on a photosensitive printing plate has been replaced by a mask formed and integrated within the printing plate. There are two techniques in the market as a method to obtain such integrated mask. One is a method to print a mask on a photosensitive plate by an inkjet printer, and the other is a method to form a mask by forming, on a photosensitive layer, a layer substantially non-transparent to ultraviolet rays (UV) (i.e., substantially blocks ultraviolet rays), and ablatable by irradiation of IR laser (this layer is generally referred to as an “IR ablation layer” etc. and is referred to as an “IR ablation layer” in the present specification), and forming an image on this layer with an IR laser. Using these techniques, an image (mask) is directly formed on a plate, and ultraviolet rays are irradiated through this image (mask) in the next step to afford plate making.
The CTP technique is convenient in that it does not require a negative film and can afford a resolution far higher than that by a conventional technique using a negative film. There is a detailed discussion on the superiority of the CTP technique to conventional technique.
In a photosensitive plate having the above-mentioned IR ablation layer, the IR ablation layer is generally made from a composition containing a polymer binder containing carbon black in a large amount (e.g., Japanese Patent No. 2916408 (page 1, claims)). In general, a cover film is formed on an IR ablation layer for protection of the IR ablation layer during preservation of the plate and handling thereof, and this cover film is removed before irradiation of IR laser or after irradiation of IR laser (generally after main exposure and before development). As a result of the investigation made by the present inventors and the like, however, it has been found that an IR ablation layer gets broken (damaged) or scratched when cover film is peeled off, which exerts an adverse influence on the finally obtained printing image. In addition, printing cost becomes higher because carbon black in the IR ablation layer transfers to and stain a developer during developing with the developer after a main exposure (irradiation of ultraviolet rays) after IR ablation, which necessitates exchange of the developer each time a plate is made.